1. Field of the Invention
The present invention relates to motor-driven printing apparatus, and in particular relates to techniques for correlating the printing stylus with the motor shaft position, for damping the printer motor to avoid unwanted oscillations at the end of each motor movement, and for providing a smooth transition of the motor from the step mode to the slew mode of operation.
2. Description of the Prior Art
Stepping motors are employed in a wide variety of applications. These motors utilize a plurality of independent windings, each winding being capable of rotating the motor shaft through an increment of a complete cycle.
In particular, such multiphase stepping motors are employed with serial printers to drive the printing stylus along a multiple position path, each position along the path corresponding to a single motor step. In U.S. Pat. Nos. 3,685,629 to Rott, and 3,700,807 to Drapeau, both of which are assigned to the assignee of the present invention, there is disclosed an impactless printer employing a stepping motor-driven stylus having a column of electrodes adapted to contact a treated paper. The stylus electrodes are selectively energized during transverse movement of the stylus across the paper to form a character at each position along the path of the stylus. Character information to the stylus electrode has heretofore been clocked at a rate which assumes that a portion of the motor step has a constant velocity. However, the stepping motor is often subjected to variations in operating voltage, motor temperature, friction, and a variety of other factors. When such occurs, the motor pulse response changes and the resulting printed character is often distorted.
The stepping motors employed in printing apparatus like that described above are high inertia, low friction components, such stepping motors tend to exhibit relatively large shaft oscillations at the end of each step, due to this low friction characteristic. If the shaft oscillations from any given step continue during the printing of a character in the next succeeding motor step, a distorted character will also result.
Techniques have been developed in the prior art for damping the above-described oscillations by applying a reverse pulse to a previously energized motor phase to brake the shaft. However, such braking pulses are not generally correlated to the shaft position, and therefore may not occur at an optimum time.
Another stepping motor control arrangement employs a shaft-mounted disc having striations thereon, and a photodetector circuit to correlate control signals to the motor with the position of the shaft. Examples of this arrangement are described in the following U.S. Pat. Nos.: 3,353,076 to Haines; 3,324,369 to Markakis; 3,345,547 to Dunne; 3,424,967 to Keller; 3,549,975 to Ferris; 3,484,666 to Easton; 3,328,662 to Gambil; 3,311,803 to Schulz; 3,510,742 to Pooley and 3,660,746 to Milek.